Facile Activation of Dihydrogen by a Phosphinito-Bridged Pt(I)−Pt(I

Mar 11, 2010 - Dipartimento di Ingegneria delle Acque e di Chimica del Politecnico di Bari, Via Orabona 4,. I-70125 Bari, Italy, Dipartimento di Scien...
0 downloads 0 Views 2MB Size
Download PDF
Published on Web 03/11/2010

Facile Activation of Dihydrogen by a Phosphinito-Bridged Pt(I)-Pt(I) Complex Piero Mastrorilli,*,† Mario Latronico,† Vito Gallo,† Flavia Polini,† Nazzareno Re,‡ Alessandro Marrone,‡ Roberto Gobetto,§ and Silvano Ellena§ Dipartimento di Ingegneria delle Acque e di Chimica del Politecnico di Bari, Via Orabona 4, I-70125 Bari, Italy, Dipartimento di Scienze del Farmaco, UniVersita` “G. d’Annunzio”, Via dei Vestini 31, 66100 Chieti, Italy, and Dipartimento di Chimica I.F.M., UniVersita` di Torino, Via P. Giuria 7, 10125 Torino, Italy Received November 17, 2009; E-mail: [email protected]

Abstract: The phosphinito-bridged Pt(I) complex [(PHCy2)Pt(µ-PCy2){κ2P,O-µ-P(O)Cy2}Pt(PHCy2)](Pt-Pt) (1) reversibly adds H2 under ambient conditions, giving cis-[(H)(PHCy2)Pt1(µ-PCy2)(µ-H)Pt2(PHCy2){κPP(O)Cy2}](Pt-Pt) (2). Complex 2 slowly isomerizes spontaneously into the corresponding more stable isomer trans-[(PHCy2)(H)Pt(µ-PCy2)(µ-H)Pt(PHCy2){κP-P(O)Cy2}](Pt-Pt) (3). DFT calculations indicate that the reaction of 1 with H2 occurs through an initial heterolytic splitting of the H2 molecule assisted by the phosphinito oxygen with breaking of the Pt-O bond and hydrogenation of the Pt and O atoms, leading to the formation of the intermediate [(PHCy2)(H)Pt(µ-PCy2)Pt(PHCy2){κP-P(OH)Cy2}](Pt-Pt) (D), where the two split hydrogen atoms interact within a six-membered Pt-H · · · H-OsP-Pt ring. Compound D is a labile intermediate which easily evolves into the final dihydride complex 2 through a facile (9-15 kcal mol-1, depending on the solvent) hydrogen shift from the phosphinito oxygen to the Pt-Pt bond. Information obtained by addition of para-H2 on 1 are in agreement with the presence of a heterolytic pathway in the 1 f 2 transformation. NMR experiments and DFT calculations also gave evidence for the nonclassical dihydrogen complex [(PHCy2)(η2-H2)Pt(µ-PCy2)Pt(PHCy2){κP-P(O)Cy2}](Pt-Pt) (4), which is an intermediate in the dehydrogenation of 2 to 1 and is also involved in intramolecular and intermolecular exchange processes. Experimental and DFT studies showed that the isomerization 2 f 3 occurs via an intramolecular mechanism essentially consisting of the opening of the Pt-Pt bond and of the hydrogen bridge followed by the rotation of the coordination plane of the Pt center with the terminal hydride ligand.

Introduction

The activation of molecular hydrogen under mild conditions is a topic of paramount importance in catalysis,1 hydrogen storage,2 and biochemistry.3 While for catalysis purposes the binding of H2 to the complexes should be tight enough for allowing subsequent H transfer onto suitable organic (or inorganic) substrates, the application in hydrogen storage requires H2 additions to be reversible.4 It is well-known that the activation of dihydrogen by transition-metal complexes proceeds through the formation of a σ adduct5 which can evolve either into a dihydrido complex †

Politecnico di Bari. Universita` “G. d’Annunzio”. Universita` di Torino. (1) (a) Handbook of Homogeneous Hydrogenation; de Vries, J. G., Elsevier, C. J., Eds.; Wiley-VCH: Weinheim, Germany, 2007. (b) Nishimura, S. Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis; Wiley: Chichester, U.K., 2001. (2) (a) Yang, J.; Sudik, A.; Wolverton, C.; Siegel, D. J. Chem. Soc. ReV. 2010, 39, 656. (b) Kubas, G. J. J. Organomet. Chem. 2009, 694, 2648. (c) van den Berg, A. W. C.; Otero Area´n, C. Chem. Commun. 2008, 668. (d) Dinca˜, M.; Long, J. R. Angew. Chem., Int. Ed. 2008, 47, 6766. (e) Orimo, S.; Nakamori, Y.; Eliseo, J. R.; Zuttel, A.; Jensen, C. M. Chem. ReV. 2007, 107, 4111. (3) Heinekey, D. M. J. Organomet. Chem. 2009, 694, 2671, and references therein. (4) Weller, A. S.; McIndoe, J. S. Eur. J. Inorg. Chem. 2007, 4411. ‡ §

4752

9

J. AM. CHEM. SOC. 2010, 132, 4752–4765

Scheme 1. Activation of H2 by Transition-Metal Complexesa

a

B is a Lewis base which can be part of an internal ancillary ligand.

(deriving from oxidative addition of the bonded H2, Scheme 1a) or into a monohydrido complex (stemming from deprotonation of the σ complex by a suitable base,6 Scheme 1b). In this framework, the majority of the studies have dealt with mononuclear metal complexes, while relatively few examples involving polynuclear metal complexes have been described.4,7 We have recently described the synthesis of the unsymmetrical phosphinito-bridged Pt(I) complex [(PHCy2)Pt(µ(5) (a) Kubas, G. J. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6901. (b) Heinekey, D. M.; Oldham, W. J., Jr. Chem. ReV. 1993, 93, 913. (c) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789. (6) Crabtree, R. H.; The Organometallic Chemistry of the Transition Metals, 4th ed.; Wiley-Interscience: Hoboken, NJ, 2005. (7) Adams, R. D.; Captain, B. Acc. Chem. Res. 2009, 41, 409. 10.1021/ja909747r  2010 American Chemical Society

H2 Activation by a Dinuclear Pt(I) Complex

ARTICLES

Scheme 2. Reaction Course for the Hydrogenation of 1

PCy2){κ2P,O-µ-P(O)Cy2}Pt(PHCy2)](Pt-Pt) (1),8 which represents a potential candidate for H2 activation under mild conditions, as it possesses an electrophilic center, the O-bound Pt atom,9 capable of forming the σ adduct, as well as a potential base, the dicyclohexylphosphinite group, in close proximity to the hypothetical reactive center. Here we describe the addition of dihydrogen to 1, which proceeds in two steps: a facile and reversible uptake of H2 to give the cis-dihydrido complex 2 and a consecutive irreversible slow isomerization into the corresponding trans isomer 3 (Scheme 2). Addition of H2 to 1

Bubbling H2 under ambient conditions into a CH2Cl2 solution of 1 caused a complete conversion after 20 min of the starting material into cis-[(H)(PHCy2)Pt(µ-PCy2)(µ-H)Pt(PHCy2){κPP(O)Cy2}](Pt-Pt) (2), the first example of a diplatinum polyhydride species generated by direct addition of molecular hydrogen to a dinuclear complex.10 The structure of 2 was determined by multinuclear NMR spectroscopy. The 31P{1H} NMR spectrum in CH2Cl2 of 2 shows four signals, each flanked by 195Pt satellites, centered at δ 119.2, 75.3, 11.3, and 9.5, respectively (Figure S1). Of these, the signal at δ 119.2 is attributable to the bridging phosphanide subtending a Pt-Pt bond (P1), having in trans positions the phosphinito ligand (2JP(1),P(3) ) 245 Hz) and the terminal hydrido ligand (2JP(1),H(4) )122 Hz, from the 31P NMR spectrum, Figure S2). The signal at δ 75.3 is ascribed to the κP-dicyclohexylphosphinito ligand, whereas the remaining two signals are attributable to the coordinated dicyclohexylphosphanes in cis positions with respect to the bridging phosphanide. In the 1H NMR spectrum in CD2Cl2 (Figure S3), the bridging hydride shows a multiplet centered at δ -4.19 flanked by three sets of 195 Pt satellites from which the coupling constant values of 1 JH(3),Pt(1) ) 544 Hz and 1JH(3),Pt(2) ) 591 Hz could be extracted, while the terminal hydride (δ -1.42), strongly coupled only (8) Gallo, V.; Latronico, M.; Mastrorilli, P.; Nobile, C. F.; Suranna, G. P.; Ciccarella, G.; Englert, U. Eur. J. Inorg. Chem. 2005, 4607. (9) Gallo, V.; Latronico, M.; Mastrorilli, P.; Polini, F.; Re, N.; Englert, U. Inorg. Chem. 2008, 47, 4785. (10) Diplatinum polyhydrides were obtained by reaction of the following mononuclear complexes with molecular hydrogen. (a) [Pt(C2H4)2(PCy3)]: Green, M.; Howard, J. A. K.; Proud, J.; Spencer, J. L.; Stone, F. G. A.; Tsipis,C.A.J.Chem.Soc.,Chem.Commun1976,671.(b)[Pt(C2O4)(PEt3)2]: Paonessa, R. S.; Trogler, W. C. Inorg. Chem. 1983, 22, 1038. (c) [{(C2F5)PCH2CH2P(C2F5)2}PtMe(O2CCF3)]: Bennet, B. L.; Roddick, D. M. Inorg. Chem. 1996, 35, 4703. Polyhydrido diplatinum complexes are also described in: (d) Leoni, P.; Manetti, S.; Pasquali, M. Inorg. Chem. 1995, 34, 749, and references therein. (e) Schwartz, D. J.; Andersen, R. A. J. Am. Chem. Soc. 1995, 117, 4014. (f) Albinati, A.; Bracher, G.; Carmona, D.; Jans, J. H. P.; Klooster, W. T.; Koetzle, T. F.; Macchioni, A.; Ricci, J. S.; Thouvenot, R.; Venanzi, L. M. Inorg. Chim. Acta 1997, 265, 255. (g) Mastrorilli, P.; Palma, M.; Fanizzi, F. P.; Nobile, C. F. Dalton Trans. 2000, 4272. (h) Mastrorilli, P.; Nobile, C. F.; Fanizzi, F. P.; Latronico, M.; Hu, C.; Englert, U. Eur. J. Inorg. Chem. 2002, 1210. (i) Bandini, A. L.; Banditelli, G.; Manassero, M.; Albinati, A.; Colognesi, D.; Eckert, J. Eur. J. Inorg. Chem. 2003, 3958. (j) Itazaki, M.; Nishihara, Y.; Osakada, K. Organometallics 2004, 23, 1610. (k) Novio, F; Gonza´les-Duarte, P; Lledo´s; Mas-Balleste´, R Chem. Eur. J. 2007, 13, 1047.

with 195Pt1 (1JH(4),Pt(1) ) 952 Hz), gives a doublet of doublets due to the couplings with the trans-P1 (2JH(4),P(1) ) 118 Hz) and with the cis-P2 (2JH(4),P(2) ) 27 Hz). In the 1H{31P} NMR spectrum, H3 and H4 appear as singlets (Figure 1), indicating that H-P scalar couplings are responsible for the multiplicity of the hydride signals in the 1H NMR spectrum. The 195Pt NMR signals (Figures S4 and S5) were found at δ -5901 (Pt1) and δ -5584 (Pt2) and are in line with those found for other systems having the Pt2(µ-H)(µ-PR2) core.11 In the IR spectrum (KBr) the terminal and bridging hydrides gave stretching bands at 1935 cm-1 (medium) and 1636 cm-1 (weak), respectively. The rate of hydrogenation for reactions in solution was found to be strongly dependent on the solvent. In fact, when the hydrogenation of 1 was carried out in aromatic solvents (toluene, benzene-d6), the complete disappearance of 1 was achieved within 6 h, a time at which the reaction mixture was comprised of 2 and 3 in an approximately 2:1 ratio. A faster and more selective reaction occurred when ethanol was present in the reaction medium. Notably, the rate of H2 uptake by 1 paralleled the amount of ethanol present in the mixture with the aromatic solvent, reaching a maximum (5 min necessary to convert completely 1 into 2) for reactions carried out in pure ethanol (in this case the initial mixture was heterogeneous, due to the scarce solubility of 1 in ethanol). Complex 2 was found to spontaneously isomerize into the more stable form trans-[(PHCy2)(H)Pt(µ-PCy2)(µ-H)Pt(PHCy2){κP-P(O)Cy2}](Pt-Pt) (3) (Scheme 2), whose structure was unambiguously assigned on the basis of HR ESI-MS and multinuclear NMR data (Figures S8-S11). The 2 f 3 isomerization occurred both in solution and in the solid state. In dichloromethane the complete isomerization took 3 days at room temperature, and while it was stored in the solid state (under an N2 atmosphere) 2 transformed after 6 days into a mixture containing 90% of 3 and 10% of 2. The formation of 2 is reversible: putting solid 2 under high vacuum at 323 K gave, after 3 h, the starting material 1 in 80% yield, the remainder being the trans-dihydride 3.12 Platinumbased compounds able to reversibly bind dihydrogen are [Pt2H(µ-dppm)2]PF6,13 [Pt2H(CO)(µ-dppm)2]PF6(Pt-Pt),14 [Pt4(PBut3)3{PBut2(CMe2CH2)}]+,15 platinum-gold clusters such as [Pt(AuPPh3)8]+,16 and clusters containing Pt(PBut3) groups.7 Differently from 2, complex 3 was stable to dehydrogenation when put under vacuum at 323 K even after 6 days. Moreover, (11) Mastrorilli, P. Eur. J. Inorg. Chem. 2008, 4835. (12) The dehydrogenation of 2 to give 1 occurred also at room temperature but was significantly slower. For example, after 4 days a mixture consisting of 3 (50%), 2 (35%), and 1 (15%) was obtained. (13) (a) Foley, H. C.; Morris, R. H.; Targos, T. S.; Geoffroy, G. L. J. Am. Chem. Soc. 1981, 103, 7337. (b) Hill, R. H.; De Mayo, P.; Puddephatt, R. J. Inorg. Chem. 1982, 21, 3642. (14) Fisher, J. R.; Mills, A. J.; Sumner, S.; Brown, M. P.; Thomson, M. A.; Puddephatt, R. J.; Frew, A. A.; Manojlovic-Muir, L.; Muir, K. W. Organometallics 1982, 1, 1421. (15) Goodfellow, R. J.; Hamon, E. M.; Howard, J. A. K.; Spencer, J. L.; Turner, D. G. J. Chem. Soc., Chem. Commun. 1984, 1604. (16) (a) Aubart, M. A.; Pignolet, L. H. J. Am. Chem. Soc. 1992, 114, 7901. (b) Aubart, M. A.; Chandler, B. D.; Gould, R. A. T.; Krogstad, D. A.; Schoondergeng, M. F. J.; Pignolet, L. H. Inorg. Chem. 1994, 33, 3724. J. AM. CHEM. SOC.

9

VOL. 132, NO. 13, 2010

4753

ARTICLES

Mastrorilli et al. Scheme 3. D2/H2 Exchange in 2

Figure 1.

1

H{31P} NMR spectra of 2 (hydride region, 298 K, CDCl3).

keeping a toluene solution of 3 at 363 K for 90 min resulted only in the onset of decomposition, but neither 1 nor 2 was formed. It is worth noting that 2 (as well as 3) was resistant to deprotonation when treated with a concentrated NaOH solution, incontrasttotherelated[(X)(PHCy2)Pt(µ-PCy2)(µ-H)Pt(PHCy2){κPP(O)Cy2}](Pt-Pt) compounds (X ) Cl, Br), which were smoothly transformed into 1. In fact, while quantitative transformation of [(X)(PHCy2)Pt(µ-PCy2)(µ-H)Pt(PHCy2){κPP(O)Cy2}](Pt-Pt) into 1 was observed after 24 h (X ) Cl) or 48 h (X ) Br) reaction,17 no appreciable reaction occurred after 3 days when 2 was reacted with NaOH(conc). Since no deprotonation was observed for the cis-[(X)(PHCy2)Pt(µ-PCy2)(µH)Pt(PHCy2){κP-P(O)Cy2}](Pt-Pt) compound when X ) I,18 it may be argued that the ability of the X ligand as a leaving group plays a role in the effectiveness of the deprotonation reaction. Treating complexes 2 and 3 with an equimolar amount of HBF4 (as the dimethyl ether adduct) resulted in the smooth protonation of the phosphinito oxygen to give the corresponding species cis- and trans-[(PHCy2)(H)Pt(µ-PCy2)(µ-H)Pt(PHCy2){κPP(OH)Cy2}]BF4(Pt-Pt) as a result of the higher basicity, in our system,17 of the PO ligand compared to that of the Pt or of the hydride centers.19 The addition of D2 to 1 to give 2-d2 was slower than the addition of H2 under the same conditions, and a kinetic isotope effect (KIE) of 1.4 was determined. This value is in line with those obtained for the addition of H2 to [Pt3Re2(CO)6(PBut3)3] (KIE ) 1.3)20 or to [Pt2Re2(CO)7(PBut3)2(µ-H)2] (KIE ) 1.5).21 Variable amounts of 2-d2 were detected by 2H and 31P NMR also when 2 was left in CH2Cl2 solution under a D2 atmosphere. Interestingly, isotopic exchange occurred also in the solid state when solid 2 was left under sonication in a D2 atmosphere at room temperature for 2 days. A plausible mechanism that explains such a behavior involves the intermediacy of the nonclassical dihydrogen complex [(PHCy2)(η2-H2)Pt(µPCy2)Pt(PHCy2){κP-P(O)Cy2}](Pt-Pt) (4), in equilibrium with 2, capable of exchanging D2 for H2 (Scheme 3). Conversely, when 2-d2 was left in CD2Cl2 solution under a H2 atmosphere, the appearance of the hydridic 1H NMR signals of 2 was detected. The σ-bonded dihydrogen complex 4 might also be (17) Latronico, M.; Polini, F.; Gallo, V.; Mastrorilli, P.; Calmuschi-Cula, B.; Englert, U.; Re, N.; Repo, T.; Raisanen, M. Inorg. Chem. 2008, 47, 9779. (18) Latronico, M.; Gallo, V.; Mastrorilli, P. Unpublished results. (19) MainNMRfeaturesofcis-[(P1HCy2)(H1)Pt(µ-P2Cy2)(µ-H2)Pt(P3HCy2){κP4P(OH3)Cy2}]+(Pt-Pt) (298 K, CD2Cl2): δP(1) ≈ δP(3) 11, δP(2) 132.1, δP(4) 96, 1JP(2),P(4) ) 247 Hz; δH(1)-1.42, δH(2)-4.47, δH(3) 5.56. Main NMR features of trans-[(H1)(P1HCy2)Pt(µ-P2Cy2)(µ-H3)Pt(P3HCy2){κP4P(OH3)Cy2}]+(Pt-Pt) (298 K, CD2Cl2): δP(1) 11.8, δP(2) 152.0, δP(3) 0.0, δP(4) 126.1, 1JP(2),P(4) ) 262 Hz, 1JP(1),P(2) ) 271 Hz; δH(1)-5.39, δH(2)-6.25, δH(3) 6.3. (20) Adams, R. D.; Captain, B.; Beddie, C.; Hall, M. B. J. Am. Chem. Soc. 2007, 129, 986. (21) Adams, R. D.; Captain, B.; Smith, M. D.; Beddie, C.; Hall, M. B. J. Am. Chem. Soc. 2007, 129, 5981. 4754

J. AM. CHEM. SOC.

9

VOL. 132, NO. 13, 2010

invoked as a possible intermediate in the transformation of 1 into 2 and vice versa. The Nonclassical Dihydrogen Complex 4

In the literature, platinum complexes with σ-bonded dihydrogen are rare if compared, for instance, to d6 metals22 and consist only of the mononuclear PtII species trans-[PtH(η2H2)(PR3)2]+ (R ) But,23 Pri 24) and trans-[Pt(PCy3)2L(η2-H2)]+ (L ) H, CH3, Ph).25 A Pt0 η2-dihydrogen complex was proposed to explain the dynamic process observed in solutions of [{Cy2P(CH2)nPCy2}PtH2] (n ) 3, 4).26 We gained evidence by means of NOESY-EXSY experiments that the equilibrium between 2 and the nonclassical dihydrogen complex 4 does exist and that such an equilibrium is completely shifted toward 2. Figure 2 shows a portion of the 1H{31P}NOESY-EXSY spectrum recorded for a CD2Cl2 solution of 2 at 298 K in which correlations indicating the chemical exchange between the terminal hydride (δ -1.4) and the bridging hydride (δ -4.2) are present. Preliminarily, it is advisable to recall that the cross peaks between the signals of the mutually cis hydrides are the result of two contributions with opposite sign. In fact, the sign of the diagonal peaks being positive, a negative contribution comes from the dipolar coupling between the two hydride nuclei in close spatial proximity, and a positive contribution is generated by their chemical exchange. Thus, the sign of the cross peaks depends on the relative intensity of the exchange over the dipolar coupling contributions: a positive sign of the cross peak is the result of a dynamic process with an exchange frequency sufficiently high to override the dipolar coupling contribution, while a negative sign of the cross peak has to be expected when the dipolar coupling contribution prevails. In the spectrum shown in Figure 2, the cross peak circled in yellow accounts for the chemical exchange between the terminal and the bridging hydrides in the isotopologue of 2 containing no 195Pt atoms (yellow box) but does not give any indication of the exchange mode: i.e., whether it is inter- or intramolecular. On the other hand, the exchange mode was ascertained to be (22) Kubas, G. J. Chem. ReV. 2007, 109, 4152. (23) Gusev, D. G.; Notheis, J. U.; Rambo, J. R.; Hauger, B. E.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1994, 116, 7409. (24) Butts, M. D.; Scott, B. L.; Kubas, G. J. J. Am. Chem. Soc. 1994, 116, 7409. (25) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Inorg. Chem. 1998, 37, 2422. (26) Clark, H. C.; Hampden Smith, M. J. J. Am. Chem. Soc. 1986, 108, 3829.

H2 Activation by a Dinuclear Pt(I) Complex

ARTICLES

observed in Figure 2. However, since the observed H2/D2 exchange can be explained only in terms of intermolecular paths, it can be concluded that its contribution to the exchange is negligible. The elusiveness of 4, which was never directly detected by NMR analyses even at low temperature, and the lack of isotope shift on passing from 2 to the deuterated complex 2-d229 concur to indicate that only a very small amount of 4 is present in equilibrium with 2. Moreover, the sharpness of the 195Pt satellites for both 1H NMR hydride signals indicates that the transformation 2 f 4 (and reverse) is slow on the NMR time scale. Accordingly, a significant difference was found in the T1 values for the bridging hydride (T1 ) 411 ms) and the terminal hydride (T1 ) 462 ms).30 We addressed also the possibility that complex 4 might be the precursor of 2 during the hydrogenation of 1. First of all, we tried to detect long-lived reaction intermediates by multinuclear 1D and 2D (1H and 31P EXSY) NMR experiments carried out in the first stages of the H2 uptake in benzene-d6 or in CD2Cl2 (in this case also at low T), but in all cases only signals attributable to 1 and 2 were observed. The absence of signals other than those of 1 and 2 indicates that, if involved in the hydrogenation of 1, complex 4 had to represent a shortlived intermediate that, as soon as it forms, evolves toward 2. This transformation conceivably occurs via a homolytic breaking of the H-H bond in the coordinated H2, so that it should be possible to confirm by exploiting PHIP (para-hydrogen induced polarization) induced by the use of para-H2.31 However, when para-H2 was reacted with 1 under several experimental conditions, no hyperpolarization was detected, indicating that the homolytic path leading from 4 to 2 is unlikely and that presumably 4 is not a precursor of 2. para-H2 Study 1

31

H{ P} EXSY spectrum of a CD2Cl2 solution of 2 (hydride region, T ) 298 K). NMR-active Pt atoms are denoted with an asterisk.

Figure 2.

intramolecular by the analysis of the cross peaks, circled in blue and red, generated by the isotopologues containing one 195Pt atom.27 The peaks circled in blue indicate that the terminal hydride HA is scalar coupled to the NMR-active Pt1 and preserves the coupling when its chemical identity changes into the bridging hydride HB (blue box). Moreover, the peaks circled in red are generated when the terminal hydride HA, initially bound to a non-NMR-active Pt atom, passes to the bridging position, in which it couples with the NMR-active Pt2 (red box). These findings strongly confirm the intermediacy of the σ adduct 4 in the dynamic process and can be explained by admitting that the rotation of the dihydrogen molecule, which remains coordinated to the metal center in 4, is responsible for the hydride exchange (Scheme 4).28 The alternative intermolecular path consisting of the possible decoordination and recoordination of dihydrogen would generate correlations between each central hydride signal and all of the satellite signals, giving a pattern different from that experimentally (27) The hydride exchange in the isotopologue containing both 195Pt1 and 195 2 Pt is too weak to be detected. (28) The same consideration holds for the experiment carried out in the less polar C6D6. In this case, the cross-peak sign was found to be temperature-dependent, being negative at 298 K and positive at 353 K. This behavior can be explained by admitting that at room temperature the exchange contribution is less than the dipolar coupling contribution, due to a low H3-H4 exchange rate, and that on increasing the T, the exchange frequency becomes sufficiently high to give a net positive peak.

A series of para-hydrogen experiments were performed in order to fully understand the mechanism of hydrogen addition on complex 1. It is well-known that PHIP effects can be observed only under the following conditions:32 • Spin correlation must be saved during the hydrogenation process; in other words, both para-hydrogen nuclei must be transferred into the same product during the reaction, keeping their spin-spin coupling. • The symmetry of the para-hydrogen molecule must be broken at the same stage during the reaction. • The relaxation rate of the para-hydrogenated intermediates or products must be relatively slow. Several different items can be in principle obtained by parahydrogen experiments, but the presence or the absence of hyperpolarized signals represents a valuable method to discriminate between different reaction pathways. A large number of transient species and reaction pathways detected by parahydrogen experiments is now available in the literature.33 (29) Bergamo, M.; Beringhelli, T.; D’Alfonso, G.; Mercandelli, P.; Sironi, A. J. Am. Chem. Soc. 2002, 124, 5117. (30) A fast equilibrium between 2 and 4 would result in an averaged T1 for the different types of hydrides. (31) PHIP leads to enhanced absorption and emission resonances in product NMR spectra when a reaction is done with hydrogen enriched in the para spin state, and the addition of hydrogen occurs such that spin correlation of the transferred protons is maintained throughout the reaction: Millar, S. P.; Jang, M.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chim. Acta 1998, 270, 363. (32) Natterer, J.; Bargon, J. Prog. Nucl. Magn. Reson. Spectrosc. 1997, 31, 293. J. AM. CHEM. SOC.

9

VOL. 132, NO. 13, 2010

4755

ARTICLES

Mastrorilli et al.

Scheme 4. Intramolecular Exchange between Terminal and Bridging Hydrides of 2

Scheme 5. Possible Mechanisms for the 2 f 3 Isomerization

We performed a series of para-hydrogen ALTADENA (adiabatic longitudinal transport and dissociation engenders net alignment) experiments34 on 1 in benzene-d6 solution at different temperatures in the range 298-323 K, but no polarized signals were observed. The same reaction was repeated by using as solvent benzene-d6 containing a catalytic quantity of ethanold6 (5 µL), CD2Cl2, or pure ethanol-d6, but in all cases no hyperpolarized peaks were detected. Also by performing a PASADENA (para-hydrogen and synthesis allows dramatically enchanced signal alignment) experiment35 on 1 we observed only an efficient 1 f 2 conversion without the presence of hyperpolarized signals. Two alternative hypotheses can be proposed for explaining all these results: (a) a low reaction rate and/or a high relaxation rate of the hyperpolarized species and (b) a heterolytic hydrogen cleavage. Hypothesis a can be ruled out, since the reaction rate is rather high and the relaxation times are in the range of 0.4-0.5 s (see above). Thus, the lack of hyperpolarized signals is likely due to the occurrence of a heterolytic hydrogen cleavage mechanism in the 1 f 2 transformation. Isomerization from 2 to 3

With regard to the isomerization mechanism from 2 to 3, at least three hypotheses can be put forward: (i) a dissociative pathway via tricoordinate Pt1; (ii) an intramolecular rearrangement via activation and rupture of the P2-H1 bond; (iii) a direct intramolecular rearrangement via an out-of-plane slippage of the terminal hydride (Scheme 5). In order to check the dissociative mechanism (path i) we carried out a 31P NMR analysis of a solution of 2 in toluene-d8, but neither was any trace of free PHCy2 (δP -27) found in the 31 P{1H} NMR spectrum nor were exchange cross peaks in the region of the coordinated P2HCy2 present in the 31P{1H} EXSY spectra recorded at various temperatures and various mixing times.36 These findings suggest that path i has to be discarded, (33) Duckett, S. B.; Wood, N. J. Coord. Chem. ReV. 2008, 252, 2278. (34) Pravica, M. G.; Weitekamp, D. P. Chem. Phys. Lett. 1988, 145, 255. (35) Bowers, C. R.; Weitekamp, D. P. J. Am. Chem. Soc. 1987, 109, 5541. 4756

J. AM. CHEM. SOC.

9

VOL. 132, NO. 13, 2010

in agreement with the occurrence of the isomerization also in the solid state, where dissociative processes are not favored.37 The second hypothesis envisages the exchange between H1 (the hydrogen originally bound to P2) and the terminal hydride H4, leading to the thermodynamically more stable 3. Such an exchange might occur either via a transition state containing a hypervalent phosphorus (as shown in Scheme 5) or via the oxidative addition of the P2-H1 bond to Pt1 followed by the formation of the P2-H4 bond. In any case, the isomerization of the deuterated compound 2-d2 should result in the formation of the complex trans-[(H)(PDCy2)Pt(µ-PCy2)(µ-D)Pt(PHCy2){κPP(O)Cy2}], having the coordinated P2DCy2 and the unlabeled terminal hydride. Carrying out the isomerization of 2-d2 resulted exclusively in the dideuterido complex trans-[(D)(PHCy2)Pt(µPCy2)(µ-D)Pt(PHCy2){κP-P(O)Cy2}](Pt-Pt) (3-d2), showing no “labeling” of the coordinated P2HCy2 and indicating that path ii is not operative. The intramolecular rearrangement constituting path iii seems therefore favored, and it is consistent with the observed isomerization in the solid state. Such a mechanism is difficult to prove experimentally but was confirmed by DFT studies, as discussed below. DFT Study

Density functional calculations were performed to investigate the possible mechanism for the reaction of 1 with H2 (and the reverse) and for the subsequent 2 f 3 isomerization. This study was carried out using models with methyl groups in place of cyclohexyl groups, an approximation that has been previously validated.38 For the sake of simplicity we will not distinguish between the actual cyclohexyl-substituted complexes and their methyl-substituted models in the discussion that follows. (36) EXSY experiments are informative of an exchange process even when the signals of a molecule involved in the dynamic process are too broad to be unambiguously detected in the 1D spectrum. (37) Aiming at comparing the isomerization rate in the presence and in the absence of free ligand, we have added 4 equiv of free PHCy2 to a solution of complex 2. Unfortunately, the free PHCy2 immediately reacted with 2 to give the monobridged complex cis-[(PHCy2)2(H)Pt(µPCy2)Pt(PHCy2)(H)(κP-P(O)Cy2)](Pt-Pt) (5), thus rendering the experiment uninformative.

H2 Activation by a Dinuclear Pt(I) Complex

ARTICLES

Figure 3. Optimized geometries of the cis- and trans-dihydrido complexes 2 and 3: (cyan) platinum; (purple) phosphorus; (red) oxygen; (gray) carbon; (white) hydrogen.

Geometries and Thermodynamics. Geometry optimizations performed on the final cis- and trans-dihydrido complexes 2 and 3 gave the structures depicted in Figure 3, showing geometries very close to those determined experimentally and theoretically for the analogous species [(PHCy2)(X)Pt(µ-PCy2)(µH)Pt{κP-P(O)Cy2}(PHCy2)](Pt-Pt) (X ) Cl, Br, I, PhO, PhS),17 with the substituent X in place of the terminal hydride. From a thermodynamic point of view the hydrogenation of 1 leading to the cis-dihydride 2 is exothermic, with an enthalpy gain of 13.9 kcal mol-1 in the gas phase (a higher value is calculated in polar solvent; see below) and a free energy gain of 4.5 kcal mol-1. The significantly smaller ∆G value with respect to ∆H is attributable to the unfavorable entropy corrections due to the loss of a translational degree of freedom. The trans isomer 3 is more stable than 2 by 4.8 kcal mol-1 in enthalpy and 5.3 kcal mol-1 in free energy in the gas phase (similar values are calculated in solution), in agreement with the experimentally found spontaneous isomerization of 2 to 3. Reaction Mechanism and Intermediates for the Hydrogenation Reaction of 1. Theoretical calculations have been initially

focused on the detailed pathway of the hydrogenation reaction of 1, through the characterization of the main intermediates and transition states and the calculation of the corresponding energy barriers. We first considered the formation of possible weak adducts between the H2 molecule and the complex 1. Three sites of complex 1 are in principle suitable for binding H2, i.e., each of the two metal centers and the phosphinito oxygen might be able to polarize the incoming H2 molecule.39 Actually, several (38) The reliability of this model in the reproduction of geometrical parameters has already been proved for complex 1 and derivatives, which have been structurally characterized by X-ray analysis, with calculated bond lengths and angles within 0.06 Å and 5°, respectively, from the experimental data. Gallo, V.; Latronico, M.; Mastrorilli, P.; Polini, F.; Re, N.; Englert, U. Inorg. Chem. 2008, 47, 4785 and Latronico, M.; Polini, F.; Gallo, V.; Mastrorilli, P.; Calmuschi-Cula, B.; Englert, U.; Re, N.; Repo, T.; Raisanen, M. Inorg. Chem. 2008, 47, 9779. (39) Accordingly, a Mulliken gross atomic charge of-0.82 has been calculated for the oxygen in 1. Gallo, V.; Latronico, M.; Mastrorilli, P.; Polini, F.; Re, N.; Englert, U. Inorg. Chem. 2008, 47, 4785.

Optimized geometries of the intermediates A-E: (cyan) platinum; (purple) phosphorus; (red) oxygen; (gray) carbon; (white) hydrogen.

Figure 4.

geometry optimizations starting from various orientations of H2 with respect to each of these sites allowed us to locate three stable adducts (A-C, Figure 4). Two of them, A and B, derive from a weak η1 coordination of the dihydrogen molecule to the Pt1 or Pt2 centers with the H-H axis almost perpendicular to the platinum coordination plane. The third adduct, C, shows the interaction between one atom of the H2 and the phosphinito oxygen, with the H-H axis almost lying in the platinum coordination plane. The binding energies for all of these adducts are very low, being 1-2 kcal mol-1 with respect to 1 and the H2 infinitely apart. We then considered the possible pathways for the evolution of the initially formed hydrogen-bonded adducts A and C to the final dihydride product 2, trying to identify the main intermediates and transition states. This point was investigated by calculating the energy profiles for these pathways, taking the Pt1-HA distance as reaction coordinate. As far as A is concerned, the Pt1-HA distance was varied from 3.045 Å, the value in A, to 1.72 Å, the value in 2, when the H2 molecule is completely split, optimizing all the remaining geometrical parameters at each fixed value of the reaction coordinate, obtaining a maximum in the energy profile, ca. 17 kcal mol-1 above A. A maximum with similar energy and structure was obtained from the energy profile starting from C and using again the Pt1-HA distance as reaction coordinate. A transition state search starting from the geometry of this maximum allowed us to locate a structure, TS1, 12.5 kcal mol-1 above 1 + H2 in enthalpy (20.7 kcal mol-1 in free energy) with an imaginary frequency of 238 cm-1 and a normal mode corresponding to the H2 splitting. The structure of TS1 is shown in Figure 5 and represents an early transition state in which (i) the H2 molecule is only slightly elongated (R(H-H) ) 0.77 Å), (ii) the two hydrogen atoms have approached the Pt1 and O J. AM. CHEM. SOC.

9

VOL. 132, NO. 13, 2010

4757

ARTICLES

Mastrorilli et al. Scheme 6. Enthalpy Profile (in the Gas Phase) for the Heterolytic

Mechanism of Hydrogenation of 1

Figure 5. Optimized geometries of the transition states TS1, TS2, TS4, and TS6: (cyan) platinum; (purple) phosphorus; (red) oxygen; (gray) carbon; (white) hydrogen.

atoms with incipient formation of Pt-H and O-H bonds, and (iii) the phosphinito oxygen has almost detached the Pt1 atom (R(Pt1-O) ) 2.9 Å). The IRC calculations indicated that TS1 is connected backward to A and forward to a stable dinuclear [(PHMe2)(H)Pt(µPMe2)Pt(PHMe2){κ1P-P(OH)Me2}](Pt-Pt) species, D, 10.7 kcal mol-1 below 1 + H2, where the two split hydrogens are bound to Pt1 and O atoms and interact with each other within a sixmembered Pt1sH · · · H-OsP-Pt2 ring; see Figure 4. The geometries of the transition state and the results of the IRC calculations thus indicate that the transformation of A into D consists of a heterolytic splitting of the H2 molecule assisted by the phosphinito oxygen. It is worth noting that D has the same structure of the intermediates involved in the reaction of 1 with strong hydrohalic acids HX (X ) Cl, Br, I), which have been identified by spectroscopic and theoretical studies as the dinuclear PtPt species [(PHCy2)(X)Pt(µ-PCy2)Pt(PHCy2){κP-P(OH)Cy2}](Pt-Pt), endowed with a six-membered PtsX · · · H-OsP-Pt ring.17 Given that such intermediates had been found to evolve to the bridging hydrido products [(PHCy2)(X)Pt(µ-H)(µPCy2)Pt(PHCy2){κP-P(O)Cy2}](Pt-Pt) through the migration of the O-bound hydrogen to the Pt-Pt bond, we considered the possibility that D evolved in the same way to give the dihydride product 2. The enthalpy profile for the intramolecular rearrangement from D to 2 was obtained by varying the distance between the oxygen-bound hydrogen and the midpoint of the Pt-Pt bond (taken as the reaction coordinate) from 3.35 Å, the value in D, to 1.07 Å, the final value in 2, and showed a maximum ca. 23 kcal mol-1 above D. A transition state search starting from the structure of the maximum gave a transition state, TS2, 20.6 kcal mol-1 in enthalpy above D (in gas phase). The frequency analysis gave an imaginary frequency of 928 cm-1 corresponding to the expected normal mode. The geometry of TS2 (Figure 5) shows that the shifting hydrogen atom has almost detached from the O atom (R(O-H) ) 2.28 Å) and significantly approached the Pt-Pt bond and in particular the Pt1 atom (R(Pt1-H) ) 1.66 Å and R(Pt2-H) ) 2.42 Å); interestingly, this H atom is significantly close to the terminal H on Pt1 (R(H-H) ) 1.16 Å), indicating a role for a transient η2-H2-like structure in the stabilization of this transition state. Subsequent IRC calculations showed that TS2 is connected 4758

J. AM. CHEM. SOC.

9

VOL. 132, NO. 13, 2010

backward to D and forward to a conformer of 2 with the phosphinito oxygen synplanar to the bridging hydride, 2′, 13.1 kcal mol-1 less stable than the lowest energy conformer with the oxygen atom antiperiplanar to the bridging hydride, and to which it is expected to evolve with a small barrier through rotation around the single Pt2-P3 bond (see Scheme 6). These results show that the proton transfer from the phosphinito oxygen to the Pt-Pt bond is an exothermic process with a relatively low energy barrier, indicating that the intramolecular conversion of D to 2 occurs easily at room temperature or moderately low temperatures and is therefore in agreement with the experimental results, showing no evidence of the intermediate D.40 The overall enthalpy profile calculated for the hydrogenation of 1 to 2 according to this heterolytic mechanism is reported in Scheme 6 and shows the highest barrier of 20.6 kcal mol-1 corresponding to the migration of the O-bound hydrogen in D to the Pt-Pt bond. We also considered the alternative path consisting of the “side on” approach of H2 to Pt1 in 1 with formation of an early η2-H2 intermediate that leads to the final dihydride product via a homolytic H-H bond breaking. No stable η2-dihydrogen complex could be optimized either when the phosphinito oxygen was bound to Pt1 (due to the coordinative saturation of the metal center) or when the Pt1-O bond was broken, but the O atom left around the Pt1 center. In fact, in the latter case, any optimization attempt resulted in the collapse of the initially delineating η2 structure into D, indicating that if the phosphinito oxygen is still close to the activated H2 molecule, it invariably rips a hydrogen atom away, forming a O-H bond. On the other hand, by approaching H2 to 1 after breaking of the Pt1-O bond and rotation of the P(O)Cy2 group about the Pt2sP3 bond to put the oxygen atom antiperiplanar to Pt1, a minimum corresponding to the η2-dihydrogen complex E could be optimized (Figure 4), which was found 2.4 kcal mol-1 in enthalpy above 1 + H2 (Scheme 7) and 13.1 kcal mol-1 above (40) Previous calculations on the protonation of 1 with strong hydrohalic acids such as HCl17 showed a significantly higher energy barrier for the isomerization of the six-membered platinacycle intermediate [(PHCy2)(Cl)Pt(µ-PCy2)Pt(PHCy2){κP-P(OH)Cy2}](Pt-Pt) to the hydrido compound [(PHCy2)(X)Pt(µ-PCy2)(µ-H)Pt(PHCy2){κP-P(O)Cy2}]: 28 kcal mol-1 instead of 20.6 kcal mol-1. The difference in value found for X ) Cl, H could explain why the intermediate with X ) Cl could be detected by NMR spectroscopy while that with X ) H could not.

H2 Activation by a Dinuclear Pt(I) Complex Scheme 7. Enthalpy Profile (in the Gas Phase) for the Homolytic Mechanism of Hydrogenation of 1

the intermediate D. The structure E corresponds to that of the nonclassical dihydrogen species 4 discussed above, with methyls replacing cyclohexyl groups. The Pt1 center in E shows a distorted-square-planar coordination geometry where the H2 unit occupies the coordination site left vacant by the phosphinito oxygen with the H-H vector lying in the coordination plane and a moderately elongated H-H distance of 0.93 Å. However, the formation of E requires, prior to the H2 binding, the complete detachment of the phosphinito oxygen from Pt1 in 1 and the formation of the antiperiplanar conformer 1′, which was found 6.9 kcal mol-1 higher in energy than 1. Moreover, an energy scan from 1 to 1′ along the dihedral Pt1-Pt2-P3-O angle from 0 to 180° showed a maximum around 90°, with an energy 18 kcal mol-1 above 1 + H2. Although we could not optimize a transition state for the 1 to 1′ conformer conversion starting from this geometry, because of the extreme flatness of the PES, a vibrational frequencies analysis led to only one imaginary frequency, indicating that this maximum is close to a first-order saddle point of the PES, as expected for a true transition state (TS3). We could therefore approximately estimate zero-point and thermal corrections to this point, calculating an activation enthalpy of 16.1 kcal mol-1 for the formation of 1′ from 1. We also searched for a transition state for the formation of E from 1 and H2 whereby the dihydrogen moiety assists the rotation along the dihedral Pt1-Pt2-P3-O angle. However, all our attempts were unsuccessful; indeed, any transition state search starting with a dihedral Pt1-Pt2-P-O angle below 90° (i.e., left in the quadratic region of the synplanar conformer 1) led to the transition state TS1 connected with D, while for dihedral angles around 90-100° the potential energy surface for the approach of H2 to 1, even in the presence of H2, remains quite flat, preventing any transition state optimization. The formation of E from 1 and H2 can therefore be reasonably described as occurring through the detachment of the phosphinito oxygen in 1 and the formation of the antiperiplanar conformer 1′, followed by the easy coordination of H2 to the coordinatively unsaturated Pt1 atom, and TS3, obtained as the maximum in the energy scan from 1 to 1′ evaluated before, 16.1 kcal mol-1 above 1, is therefore a good approximation for the transition state from 1 to E. The binding energy of H2 to the coordinatively unsatured Pt1 center in 1′ is relatively low (4.5 kcal mol-1 in enthalpy) and without any significant energy barrier (an energy scan led to a maximum less than 1 kcal mol-1 above 1′ + H2),

ARTICLES

leading to the overall enthalpy change of +2.4 kcal mol-1 for the formation of E from 1 and H2 (a free energy change of 11.7 kcal mol-1). An energy scan for the oxidative addition of the η2-bound H2 in E, leading to 2, showed that this is an easy process with a maximum of only 3 kcal mol-1. We could locate a transition state for this step, TS4, which has been confirmed by IRC calculations to connect E to 2. A vanishingly low activation enthalpy of 0.4 kcal mol-1 (activation free energy of 0.5 kcal mol-1) was calculated for this homolytic dihydrogen dissociation process. The overall energy profile for this homolytic path is reported in Scheme 7 and shows an energy barrier of 16.1 kcal mol-1, essentially corresponding to the breaking of the Pt-O bond in 1 and its rearrangement to the antiperiplanar conformer 1′. The comparison between the energy profiles for the heterolytic and homolytic pathways shows that, although the highest activation energies for the homolytic path are slightly lower, 16.1 vs 20.6 kcal mol-1, the much lower barrier for the initial formation of D and the unfavorable requirement that the Pt-O must first break and then rotate to an antiperiplanar position forces the mechanism to follow the heterolytic pathway. This results is consistent with the experimental finding that no PHIP was observed in 1H NMR spectra recorded during the hydrogenation of 1 carried out with para-H2. The occurrence of the heterolytic rather than the homolytic mechanism is also supported by the positive KIE of 1.4 measured for the addition of H2 to 1; indeed, the barrier for the homolytic mechanism does not involve the breaking of the H2 but that of the Pt-O bond and no KIE would be thus expected. Heterolysis of H2 mediated by transition-metal-ligand bonds have been recently reported for Rh-S,41 Ir-S,41b and Ni-N42 systems. Mechanism of 2 f 3 Isomerization

All of the isomerization mechanisms proposed in Scheme 5 were addressed, and for each of them we calculated the geometries and the energies of all involved intermediates and transition states and estimated the corresponding energy barriers. The dissociative mechanism (path i) was simulated through the dissociation of the P2HCy2 ligand from Pt1, followed by the cis to trans isomerization of the tricoordinated Pt1 center, with a T geometry, and the recoordination of the PHCy2 ligand. As expected, the first step (i.e., dissociation of the P2HCy2) was calculated to be endothermic, by 32 kcal mol-1 (Scheme 8), and a linear transit calculation, using the Pt1-P2 distance as reaction coordinate, also showed that it is essentially barrierless. Moreover, upon the detachment of PHCy2, the bridging hydride ligand spontaneously moves toward the neighboring terminal hydride, forming a Pt1-bound η2 dihydrogen complex, F (Figure S19). The high energy of 32 kcal mol-1, calculated for the detachment of the PHCy2 ligand on Pt1, and the formation of the dihydrogen complex, whose isomerization would require further unfavorable steps, allow us to discard this mechanism, in agreement with the experimental evidence showing no trace (41) (a) Ienco, A.; Calhorda, M. J.; Reinhold, J.; Reineri, F.; Bianchini, C.; Peruzzini, M.; Vizza, F.; Mealli, C. J. Am. Chem. Soc. 2004, 126, 11954. (b) Ohki, Y.; Sakamoto, M.; Tatsumi, K. J. Am. Chem. Soc. 2008, 130, 11610–11611. (42) Misumi, Y.; Seino, H.; Mizobe, Y. J. Am. Chem. Soc. 2009, 131, 14636. J. AM. CHEM. SOC.

9

VOL. 132, NO. 13, 2010

4759

ARTICLES

Mastrorilli et al.

Scheme 8. Enthalpy Profile (in the Gas Phase) for the Considered Pathways i-iii of the 2 f 3 Isomerization

of free PHCy2 and no exchange cross peak in the NMR spectra during the 2 f 3 isomerization. We also considered path ii on the basis of the exchange between H1 (the hydrogen originally bound to P2) and the terminal hydride H4 (Scheme 5). In spite of several attempts, no transition state was found for the concerted pathway consisting of the simultaneous transfer of H from the phosphane P2 to Pt1, trans to the bridging hydride, and from Pt1 to P2. On the other hand, we could characterize the asynchronous pathway consisting of an oxidative addition of the P2-H1 bond leading to a trihydride intermediate, G (Figure S20), and followed by a reductive elimination to 3 involving the newly formed terminal phosphanide and the cis hydrido ligand. G shows a pentacoordinated square-pyramidal Pt1 center with an apical terminal phosphanide and two equatorial terminal hydrides and is a quite unstable species, 28.0 kcal mol-1 above 2. We found a late transition state for this oxidative addition, TS5 (Figure S22), 30.0 kcal mol-1 above 2. This large activation enthalpy allows us to discard this mechanism, in agreement with the experimental evidence indicating that the isomerization of 2-d2 led to a final 3-d2 complex showing no labeling of the coordinated P2HCy2 ligand. We finally addressed the direct intramolecular isomerization via an out-of-plane slippage of the terminal hydride, mechanism iii. A transition state search along the direct pathway from 2 to 3 allowed us to localize a structure, TS6, 24.2 kcal mol-1 above 2, which was confirmed by an IRC calculation to be directly connected to 2 and 3. This TS showed a distorted geometry, showing broken Pt1-Pt2 and Pt1-H3 bonds, a shift of the phosphane ligand on Pt1 from cis to trans position with respect to the bridging phosphanide, and a rotation of the Pt1 and Pt2 coordination planes around the P1-Pt1 and P1-Pt2 bonds (see Figure 5). The enthalpy profiles for the above plausible pathways are reported in Scheme 8 and indicate that the direct pathway iii has the lowest activation enthalpy and is therefore expected to be the most favorable.43 The calculated enthalpy barrier of 24.2 kcal mol-1 (2-3 kcal mol-1 lower in solution, see below) is consistent with the low rate experimentally observed for the 2 f 3 isomerization. 4760

J. AM. CHEM. SOC.

9

VOL. 132, NO. 13, 2010

Scheme 9. Enthalpy Profiles in Solution for the Heterolytic Pathway of Hydrogenation

An intramolecular mechanism similar to pathway iii has been recently reported to account for the equivalence of terminal and bridging hydride ligands in the complex [Pt2(H)2(µ-H)(dppp)2]+.10k Solvent Effects. The hydrogenation mechanisms indicated by our calculations, either a heterolytic splitting of the H2 molecule or a homolytic splitting requiring the previous breaking of the polar Pt-O bond, suggest that solvent effects could be very important, as experimentally observed, with polar solvents accelerating the hydrogenation reaction. We have thus evaluated the solvent effects on the energy profiles for both hydrogenation pathways and also for the favored direct mechanism of 2 f 3 isomerization, considering the three solvents experimentally employed and spanning a wide range of polarity: i.e., toluene (ε ) 2.4), dichloromethane (ε ) 9.1), and ethanol (ε ) 24.3). The results are reported in Schemes 9-11 and indicate that an increase of the solvent polarity clearly favors both hydrogen splitting pathways, while it barely affects the isomerization process. Indeed, the thermodynamics of the hydrogenation reaction is favored by the polarity of the medium, with calculated reaction enthalpies of -13.9 kcal mol-1 (in gas phase), -17.8 kcal mol-1 (in toluene), -21.5 kcal mol-1 (in dichloromethane), and -21.4 kcal mol-1 (in ethanol). Moreover, the highest enthalpy barrier for the heterolytic pathway, the hydrogen (43) An associative mechanism for the 2 f 3 isomerization, consisting of the addition of H2 to 2 and subsequent reductive elimination of H2 to give 3, was also addressed. The addition of a further H2 molecule to Pt1 in 2 is a very slightly endothermic process, by 3 kcal mol-1, and leads to a stable tetrahydrido species with three terminal hydride ligand on the octahedral Pt1 center and a fourth hydride bridging the two metals, H (see Scheme S18). We found an early transition state for this oxidative addition, TS7, with a H-H distance of 1.02 Å, Pt1-H distances of 1.97 and 1.98 Å, and a relatively high activation energy of 28 kcal mol-1 that renders this path unlikely. IRC calculations have shown that TS7 correlates backward with the most stable isomer of H, with the terminal PHCy2 ligand in an axial position. An analogous transition state, TS8, 25 kcal mol-1 above H, was calculated for the final step, consisting of the reductive elimination of H2 from H, involving the terminal hydride cis to the bridging hydride and leading to the final trans-dihydride 3.

H2 Activation by a Dinuclear Pt(I) Complex

ARTICLES

Scheme 10. Enthalpy Profiles in Solution for the Homolytic Pathway of Hydrogenation

Scheme 11. Enthalpy Profiles in Solution for the Isomerization of 2

to 3

transfer from O to the Pt2 core in D, decreases from 20.6 kcal mol-1 (in the gas phase) to 14.9 kcal mol-1 (in toluene), 9.5 kcal mol-1 (in dichloromethane), and 9.3 kcal mol-1 (in ethanol); see Scheme 9. Scheme 10 shows that, analogously, also the highest enthalpy barrier for the homolytic pathway, the cleavage of the Pt-O bond in 1, decreases from 16.1 kcal mol-1 (in the gas phase) to 12.8 kcal mol-1 (in toluene), 9.1 kcal mol-1 (in dichloromethane) and 9.0 kcal mol-1 (in ethanol), still remaining always higher than that for the heterolytic pathway. On the other hand, the thermodynamics of the isomerization is poorly affected, the isomerization enthalpy passing from -4.8 kcal mol-1 (in the gas phase) to -4.5 kcal mol-1 (in toluene), -4.9 kcal mol-1 (in dichloromethane) and -4.3 kcal mol-1 (in

ethanol), and the isomerization activation enthalpy for the most favored direct isomerization pathway iii decreases only very slightly, being 24.2 kcal mol-1 (in the gas phase), 22.4 kcal mol-1 (in toluene), 21.1 kcal mol-1 (in dichloromethane), and 20.5 kcal mol-1 (in ethanol) (Scheme 11). These results are in agreement with the experimental findings, showing that the hydrogenation of 1 to 2 is complete in 6 h in toluene, 20 min in dichloromethane, and 5 min in ethanol, while the times required for a complete isomerization in the three solvents are roughly the same. A growing body of data indicate that alcohols can act as promoters of heterolytic H2 cleavage in several metal-catalyzed hydrogenation reactions, and this effect has been attributed either to the polarization of the cleaving H2 moiety in the transition state through a hydrogen bond or to the direct involvement of the alcohol oxygen in a proton-hopping mechanism.44 For this reason we performed the calculations on the heterolytic splitting of H2 by 1 also including one explicit EtOH molecule, considering either a simple hydrogen-bond polarization of the previously characterized transition state (path a) or a protonhopping mechanism through an eight-membered-ring transition state (path b). Scheme 12 shows paths a and b for the reaction of 1 with D2 (rather than H2) that leads to different isotopologues of the intermediate D and of the product 2, depending on the followed path: path a gives D-d2 (as EtOH adduct) and 2-d2, whereas path b gives D-dh (as the EtOD adduct) and 2-dh. Our calculations showed the formation of a quite stable adduct of 1 and EtOH, 1 · EtOH, with a binding enthalpy of 6.2 kcal mol-1. In the same way observed for 1, 1 · EtOH forms a labile adduct with the approaching H2 molecule, A · EtOH, showing a η1 coordination to Pt1, that has been characterized by a negligibly (44) (a) Sandoval, C. A.; Ohkuma, T.; Mun˜iz, K.; Noyori, R. J. Am. Chem. Soc. 2003, 125, 13490. (b) Casey, C. P.; Johnson, J. B.; Singer, S. W.; Cui, Q. J. Am. Chem. Soc. 2005, 127, 3100. (c) Comas-Vives, A.; Arellano-Gonza`les, C.; Corma, A.; Iglesias, M.; Sa`nchez, M.; Ujaque, G. J. Am. Chem. Soc. 2006, 128, 4756. J. AM. CHEM. SOC.

9

VOL. 132, NO. 13, 2010

4761

ARTICLES

Mastrorilli et al.

Scheme 12. Ethanol-Catalyzed D2 Heterolytic Splitting through (a) Polarization of the Breaking H2 Moiety in the Transition State by

Formation of a Hydrogen Bond and (b) a Proton-Hopping Mechanism via an Eight-Membered-Ring Transition State

small binding enthalpy (in the gas phase). The transition state TS1 · EtOH was found for the phosphinito-assisted heterolytic splitting of H2, which shows a geometry close to that found in the absence of ethanol (TS1) with the EtOH molecule involved in two hydrogen bonds: one to the phosphinito oxygen and the other one to the adjacent H atom (see path a in Scheme 12). An activation enthalpy of 3.4 kcal mol-1 was calculated (after the inclusion of the continuum solvent effect) which was 3.0 kcal mol-1 lower than the value calculated without the inclusion of an explicit EtOH molecule (6.4 kcal mol-1, see Scheme 9 and Table S16d). In spite of several efforts, no transition state could be found corresponding to path b in Scheme 12. An energy scan along the distance between the ethanol oxygen and the adjacent hydrogen atom of H2, from 2.4 Å (the value in A · EtOH) to 1.01 Å (the value in EtOH), led to a maximum ca. 27 kcal mol-1 above A · EtOH, suggesting that path b requires indeed a high activation energy. The unlikeliness of path b has been experimentally confirmed by the outcome of the reaction of 1 with D2 in EtOH, which gave exclusively the 2-d2 isotopologue (instead of 2-dh, the product of path b; see Scheme 12). These results indicate that ethanol favors H2 heterolytic splitting through both bulk polarization and specific hydrogen bond interactions, both effects stabilizing the transition state TS1. We also calculated the transition state, TS2 · EtOH, for the subsequent rearrangement from D to 2 by explicitly including an EtOH molecule (hydrogen bonded to the phosphinito oxygen). It was found 11.1 kcal mol-1 higher in enthalpy with respect to D, thus indicating that the activation enthalpies do not change significantly on passing from the continuum solvent model to the explicit consideration of one EtOH molecule. Reversibility of the Hydrogenation. The dehydrogenation of the hydride products 2 and 3 to give 1 was also investigated. A careful analysis of the free energies for the hydrogenation of 1 to 2 in the gas phase, and of the isomerization of 2 to 3 (see Scheme 13), showed that the free energy of 2 is only 4.5 kcal mol-1 below 1 and therefore an equilibrium between 1 and 2 could be operative, thus giving a rationale for the observed reversibility of the hydrogenation of 1 under reduced pressure. When we consider the mechanism of this dehydrogenation reaction on the basis of the calculated energy profiles (see Schemes 6 and 7) we should take into account that the highvacuum conditions could change the favored hydrogenation pathwaysthe heterolytic splitting through Dswhen the reaction is carried out in the reverse direction. Indeed, a lower activation enthalpy has been calculated for the formation of the η24762

J. AM. CHEM. SOC.

9

VOL. 132, NO. 13, 2010

Scheme 13. Free Energies of the Hydrogenation Products

dihydrogen adduct Esthe intermediate of the homolytic splitting pathwaysthan for the formation of D (16.7 kcal mol-1 instead of 23.8 kcal mol-1), and moreover, due to the lability of the η2-bound H2 molecule, E is expected to easily lose dihydrogen under vacuum conditions. The activation enthalpy for the complete dehydrogenation of 2 must, however, include also the binding enthalpy of H2 in E (4.5 kcal mol-1) and reaches a value of 21.2 kcal mol-1, comparable to that for the isomerization of 2 to 3 (24.2 kcal mol-1), so that the processes of dehydrogenation and of isomerization are expected to compete, as experimentally observed. On the other hand, the hydrogenation in solution, especially in the polar solvents CH2Cl2 and ethanol, is much more exergonic (see Scheme 13) by about 12 kcal mol-1 from 2 and 17 kcal mol-1 from 3 and the reverse reaction can be ruled out in agreement with the experimental findings. H/D Exchange and Isotopic Kinetic Effect. The hydride ligands in 2 easily undergo intermolecular exchange with D2 under atmospheric pressure at room temperature both in solution and in the solid state. The nonclassical dihydrogen complex [(PHCy2)(η2-H2)Pt(µ-PCy2)Pt(PHCy2){κP-P(O)Cy2}](Pt-Pt) (4) can be relatively easily accessed from 2 at room temperature and is therefore a good candidate as the intermediate in the hydride exchange process by either D2 or a second H2 molecule. Indeed, once 4 is formed, the labile η2-bound dihydrogen ligand can be easily lost and substituted by an incoming D2 (or H2) molecule, which then undergoes an oxidative addition to 2-d2 (or back to 2). The latter step has been already addressed within the homolytic

H2 Activation by a Dinuclear Pt(I) Complex

ARTICLES

Scheme 14. Proposed Mechanism for the H/D Exchange of 2

Scheme 15. Enthalpy Profile (in the Gas Phase) for the Hydrogen Exchange in 2

mechanism for the hydrogenation of 1 (see Scheme 7) and has been shown to pass through a low-energy transition state, TS4, with a negligible activation enthalpy (0.4 kcal mol-1). We have also considered the H2 substitution process in E (the model for 4 used in DFT calculations), which follows a dissociative mechanism with no significant transition state for the H2 detachment from E and for the D2 (or H2) rebinding. The overall mechanism is reported in Scheme 14, and the enthalpy profile in Scheme 15 shows that the highest barrier for this exchange process is 20.8 kcal mol-1 in the gas phase (21-22 kcal mol-1 in solution depending on the solvent). This activation energy value is sufficiently low to allow an easy deuterium exchange at room temperature but high enough to make the fluxionality process relatively slow on the NMR time scale, thus allowing us to observe two separate signals for the terminal and bridging hydride ligands on the 1H NMR spectra of 2 at room temperature. Finally, we calculated the KIE for the hydrogenation reaction, on the basis of the free energy profile of the stepwise heterolytic mechanism (see details in the Experimental Section) and found a value of 1.1. On the other hand, no KIE would be expected for the homolytic mechanism, because the barrier does not involve the breaking of the H2 but that of the Pt-O bond. The calculated value for the heterolytic mechanism (kH/kD ) 1.1) is qualitatively consistent with the rough experimental value of 1.4, further supporting such a mechanism. Conclusions

The Pt-O fragment of 1 constitutes the reaction core of the molecule, capable of activating H2 under ambient conditions. In fact, whatever geometry the approaching dihydrogen adopts

(i.e., side-on or end-on), the interaction of H2 with Pt1 gives directly the labile intermediate D, which evolves into 2 by H transfer from the PO to the Pt-Pt bond. A relatively low activation enthalpy (14.9 kcal mol-1 in toluene and 9.5 kcal mol-1 in dichloromethane) has been found for this heterolytic H2 splitting process. The lack of formation of a nonclassical H2 complex in the 1 f 2 transformation is related to the presence of the Pt-bonded oxygen atom, which acts as a Lewis base toward the activated H2, ripping one of the two H atoms as soon as H2 approaches the Pt1 center. Thus, a mandatory condition for the formation of the σ-bonded H2 complex 4 is that the phosphinito oxygen stays in an antiperiplanar position with respect to Pt1, a situation fulfilled in the cis-dihydride 2. Hence, complex 4 is an intermediate in the dehydrogenation of 2 to 1. The species 4 is also involved in the intramolecular exchange between terminal and bridging hydrides in 2, as well as in the intermolecular H2/D2 exchange leading from 2 to 2-d2 and vice versa. Experimental and DFT methods allowed us to determine that path iii of Scheme 5 is followed in the 2 f 3 isomerization: i.e., the intramolecular mechanism passing through the transition state TS6 with an activation enthalpy of 24.2 kcal mol-1. Further evidence of the fact that the H2 addition does not involve a σ-H2 complex was also obtained by para-H2 experiments, where the PHIP absence is in full agreement with a heterolytic process. As expected for a heterolytic mechanism of H2 activation, a strong solvent effect was experimentally found and theoretically justified by DFT. The addition of hydrogen to 1 under mild conditions (1 bar, 298 K) suggests future applications of 1 as a hydrogenation catalyst. Complex 1 might also represent a good model for hydrogen storage. Experimental Section Complex 1 was prepared as described in ref 9. A stream of dry D2 was obtained by the action of Na on CH3OD and purified by passage through a cold (liquid nitrogen) trap that retained the methanol and an H2SO4 trap that retained the moisture. Dicyclohexylphosphane was obtained commercially (Strem) and used under N2. The high-purity deuterated solvents CD2Cl2, benzene-d6, and ethanol-d6 were purchased from EURISO-TOP (Saint Aubin, France) and used without further purification. The solvents used were freshly distilled and oxygen-free. C, H elemental analyses were carried out on a Carlo Erba EA1108 CHNS-O elemental analyzer. Infrared spectra were recorded on a Bruker Vector 22 spectrometer. Mass spectrometry analyses were performed using a time-of-flight mass spectrometer equipped with an electrospray ion source (Bruker microTOF). The analyses were carried out in the positive ion mode. The sample solutions were introduced by continuous infusion with the aid of a syringe pump at a flow rate of 180 µL/min. The instrument was operated at end plate offset J. AM. CHEM. SOC.

9

VOL. 132, NO. 13, 2010

4763

ARTICLES

Mastrorilli et al.

-500 V and capillary -4500 V. The nebulizer pressure was 1.5 bar (N2) and the drying gas (N2) flow 10 L/min. Capillary exit and skimmer 1 were 120 and 40 V, respectively. The drying gas temperature was set at 220 °C. The software used for the simulations was Bruker Daltonics DataAnalysis (version 3.3). NMR spectra were recorded on a Bruker Avance 400 spectrometer; frequencies are referenced to Me4Si (1H), 85% H3PO4 (31P), and H2PtCl6 (195Pt). The coupling constants not directly extractable from the one-dimensional spectra were obtained and attributed by the tilts of the multiplets due to the “passive” nuclei45 in 1H-31P HMQC and 1 H-195Pt HMQC spectra. The 1H NOESY-EXSY spectrum reported in Figure 2 was recorded using a mixing time of 600 ms. The 2H NMR spectra were recorded by running the instrument unlocked. Computational Details. All calculations were carried out at density functional theory level by using Jaguar 6 programs.46 Geometry optimization and linear transit calculations have been performed by using the B3LYP functional47 with the 6-311G** basis set for main-group elements (C, O, H) and by the Los Alamos LACV3P*48 basis for Pt, including relativistic effective core potentials. IRC calculations49 have been further carried out in order to assess the reactant and product configurations, which are connected through the involved TS. Subsequent vibrational frequency calculations based on analytical second derivatives at the B3LYP/LACV3P** level of theory have been carried out to confirm the nature of the local minima and transition state and to compute the zero point energy (ZPE) and vibrational entropy corrections at 298.15 K. Solvation free energies were evaluated by a selfconsistent reaction field (SCRF) approach50 based on accurate numerical solutions of the Poisson-Boltzmann equation.51 Solvation calculations were carried out on the gas-phase geometry at the B3LYP/LACV3P** level of theory by simulating the reaction field of three solvents: toluene (ε ) 2.4 and σ ) 2.762 Å; dielectric constant and solvent radius, respectively), dichloromethane (ε ) 8.93 and σ ) 2.670 Å), and ethanol (ε ) 24.3 and σ ) 2.262 Å). The activation and reaction enthalpies in solution have been estimated by adding the solvation free energy to the corresponding gas-phase enthalpy. Their variations are expected to be reliable, since the solvation entropy variations are likely to be negligible. The kinetic isotope effect (KIE) of the hydrogenation reaction was determined using the DFT data based on the semiclassical model.52 This model uses Eyring’s equation for the reaction rate constant kr of an elementary step as a function of temperature T:

kr )

kBT -∆Gq/RT e h

where ∆Gq is the free energy of activation, kB is the Boltzmann constant, and h is the Planck constant. On the basis of this equation, the KIE of such an elementary step can be calculated from the free energies of activation of the deuterium substituted and reference systems as (45) Carlton, L. Bruker Rep. 2000, 148, 28. (46) Jaguar, version 6.0; Schro¨dinger, LLC, New York, 2005. (47) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. 1988, 37, 785. (48) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (49) (a) Gonzalez, C.; Schegel, H. B. J. Chem. Phys. 1989, 90, 2154. (b) Gonzalez, C.; Schegel, H. B. J. Chem. Phys. 1990, 94, 5523. (50) (a) Tomasi, J.; Persico, M. Chem. ReV. 1994, 94, 2027. (b) Cramer, C. J.; Truhlar, D. G. Chem. ReV. 1999, 99, 2161. (51) (a) Tannor, D. J.; Marten, B.; Murphy, R. B.; Friesner, R. A.; Sitkoff, D.; Nicholls, A.; Ringnalda, M. N.; Goddard, W. A., III; Honig, B. J. Am. Chem. Soc. 1994, 116, 11875. (b) Marten, B.; Kim, K.; Cortis, C.; Friesner, R. A.; Murphy, R. B.; Ringnalda, M. N.; Sitkoff, D.; Honig, B. J. Phys. Chem. 1996, 100, 11775. (52) Melander, L.; Saunders, W. H., Jr. Reaction Rates of Isotopic Molecules; Robert E. Krieger Publishing: Malabar, FL, 1987. 4764

J. AM. CHEM. SOC.

9

VOL. 132, NO. 13, 2010

KIEr ) kH /kD ) e-(∆G H-∆G D)/RT q

q

A two-step process such as the hydrogenation of 1 occurring via the heterolytic mechanism in Scheme 6 can be described by the following steady-state analysis: k1

k2

AaDf2 k-1

where an irreversible hydrogen shift from O to the Pt-Pt bond was assumed, consistent with the experimental evidence and the calculated profile in CH2Cl2 solution, and the observed rate constant kobs can be written as

kobs )

k1k2 k-1 + k2

The observed KIE can be therefore expressed as

KIEobs )

H kobs D kobs

)

kH1 kH2

D k-1 + kD2

H k-1 + kH2

kD1 kD2

) KIE1KIE2

D k-1 + kD2 H k-1 + kH2

cis-[(PHCy2)(H)Pt(µ-PCy2)(µ-H)Pt(PHCy2){KP-P(O)Cy2}](Pt-Pt) (2). In a Schlenk tube containing a dichloromethane solution of 1 (80 mg, 0.066 mmol in 0.8 mL), H2 was bubbled under ambient conditions at a flow rate of 0.8 mL/min. After 20 min the solvent was removed under reduced pressure. The pale yellow residue was not kept under high vacuum in order to avoid its transformation into 1. Yield: 77.8 mg (97%). The complex is soluble in halogenated solvents, in aromatic solvents, and in ethanol; it is poorly soluble in n-hexane. IR (KBr, pellet): 2923 (s); 2848 (s); 2658 (w); 2301 (w), ν(P-H); 1935 (m), ν(H-Pt); 1636 (w), ν(Pt-H-Pt); 1447 (s); 1342 (w); 1328 (w), 1292 (w); 1265 (m); 1190 (m); 1176 (m); 1105 (m); 1049 (s), ν(PdO); 1002 (s); 916 (m); 863 (m); 848 (m); 818 (m); 736 (s); 538 (m); 516 (s); 456 (m); 414 (w); 375 (w) cm-1. 1H NMR (C6D6, 298 K): δ 5.34 (m, H(1), 1JH(1),P(2) ) 370 Hz, 2JH(1),Pt(1) ) 64 Hz), 5.02 (m, H(2), 1JH(2), P(4) ) 340 Hz, 2JH(2),Pt(2) ) 52 Hz), -0.86 (m, H(4), 2JH(4),P(1) ) 122 Hz, 2JH(4),P(2) ) 27 Hz, 1JH(4),Pt(1) ) 948 Hz), -3.63 (m, H(3), 2 JH(3),P(1) ) 16 Hz, 2JH(3),P(2) ) 71 Hz, 2JH(3),P(3) ) 10 Hz, 2JH(3),P(4) ) 68 Hz,1JH(3),P(t1) ) 544 Hz, 1JH(3),Pt(2) ) 591 Hz) ppm. 31P{1H} NMR (CD2Cl2, 298 K): δ 119.2 (dd, P(1), 2JP(1),P(3) ) 245 Hz, 2JP(1),P(2) ) 11 Hz, 1JP(1),P(t1) ) 1611 Hz, 1JP(1),Pt(2) ) 1120 Hz), 75.3 (dd, P(3), 2JP(3),P(1) ) 245 Hz, 2JP(3),P(4) ) 22 Hz, 1JP(3),Pt(2) ) 2483 Hz), 11.3 (br, P(4), 2 JP(4),P(1) ) 11 Hz, 3JP(4),P(2) ) 49 Hz, 2JP(4),P(3) ) 21 Hz, 1JP(4),Pt(2) ) 3917), 9.5 (dd, P(2), 2JP(2),P(1) ) 11 Hz, 3JP(2),P(4) ) 49 Hz, 1JP(2),Pt(1) ) 3944 Hz, 2JP(2),Pt(2) ) 202 Hz) ppm. 195Pt{1H} NMR (C6D6, 298 K): δ -5901 (dddd, Pt(1), 1JPt(1),P(1) ) 1601 Hz, 1JPt(1),P(2) ) 3967 Hz, 2JPt(1),P(3) ) 36 Hz, 2JPt(1),P(4) ) 201 Hz, 1JPt(1),Pt(2) ) 608 Hz), -5584 (dddd, Pt(2), 1JPt(2),P(1) ) 1110 Hz, 2JPt(2),P(2) ) 204 Hz, 1JPt(2),P(3) ) 2486 Hz, 1 JPt(2),P(4) ) 3896 Hz, 1JPt(2),Pt(1) ) 608 Hz) ppm. trans-[(H)(PHCy2)Pt(µ-PCy2)(µ-H)Pt(PHCy2){KP-P(O)Cy2}](Pt-Pt) (3). In a Schlenk tube containing a dichloromethane solution of 1 (60 mg, 0.05 mmol in 0.8 mL), H2 was bubbled under ambient conditions at a flow rate of 0.8 mL/min for 30 min. The solution was left to stand 3 days at room temperature; then the solvent was removed under high vacuum and pure 3 was obtained as a yellow solid. Yield: 58.9 mg (98%). The complex is soluble in halogenated solvents, in aromatic solvents, and in ethanol; it is insoluble in n-hexane. Anal. Calcd for C48H92OP4Pt2: C, 48.1; H, 7.73. Found: C, 47.9; H, 7.82. HR ESI-MS: exact mass for C50H95NOP4Pt2 m/z 1198.5394; measured m/z 1199.5498 (M + H)+. IR (KBr, pellet): 2931 (s); 2849 (s); 2653 (w); 2305 (m), ν(P-H); 2089 (m), ν(H-Pt); 1980 (w); 1824 (w); 1711 (w); 1635 (w), ν(Pt-H-Pt); 1597 (w); 1447 (s); 1342 (m); 1327 (m), 1292 (m); 1263 (s); 1191 (m); 1177 (m); 1107 (s); 1076 (s); 1045 (s), ν(PdO); 1002 (s), 916 (m); 886 (m); 849 (m); 817 (m); 735 (m); 668 (w); 575 (w); 521 (m); 467 (m);

H2 Activation by a Dinuclear Pt(I) Complex

453 (m); 416 (w); 382 (w) cm-1. 1H NMR (C6D6, 298 K): δ 5.29 (m, H(2), 1JH(2),P(4) ) 371 Hz, 2JH(2),Pt(2) ) 42 Hz), 4.80 (m, H(1), 1 JH(1),P(2) ) 318 Hz, 2JH(1),Pt(1) ) 16 Hz), -5.29 (m, H(4), 2JH(4),P(1) ) 4 Hz, 2JH(4),P(2) ) 26 Hz, 3JH(4),P(3) ) 5 Hz, 3JH(4),P(4) ) 35 Hz,1JH(4),Pt(1) ) 1499 Hz, 2JH(4),Pt(2) ) 118 Hz), -5.40 (m, H(3), 2 JH(3),P(1) ) 18 Hz, 2JH(3),P(2) ) 11 Hz, 2JH(3),P(3) ) 16 Hz, 2JH(3),P(4) ) 88 Hz,1JH(3),Pt(1) ) 364 Hz, 1JH(3),Pt(2) ) 632 Hz) ppm. 31P{1H} NMR (C6D6, 298 K): δ 134.5 (pseudo td, P(1), 2JP(1),P(2) ) 257 Hz, 2 JP(1),P(3) ) 263 Hz, 2JP(1),P(4) ) 16 Hz, 1JP(1),Pt(1) ) 2066 Hz, 1JP(1),Pt(2) ) 1193 Hz), 83.3 (d, P(3), 2JP(3),P(1) ) 263 Hz, 1JP(3),Pt(2) ) 2599 Hz), 14.4 (d, P(2), 2JP(2),P(1) ) 257 Hz, 1JP(2),Pt(1) ) 2564 Hz), 11.5 (br, P(4), 2JP(4),P(1) ) 16 Hz,2JP(4),Pt(1) ) 170 Hz, 1JP(4),Pt(2) ) 3756 Hz) ppm. 195Pt{1H} NMR (C6D6, 298 K): δ -5947 (dddd, Pt(1), 1 JPt(1),P(1) ) 2068 Hz, 1JPt(1),P(2) ) 2561 Hz, 2JPt(1),P(3) ) 60 Hz, 2 JPt(1),P(4) ) 170 Hz, 1JPt(1),Pt(2) ) 1340 Hz), -5643 (dddd, Pt(2), 1 JPt(2),P(1) ) 1193 Hz, 2JPt(2),P(2) ) 10 Hz, 1JPt(2),P(3) ) 2581 Hz, 1 JPt(2),P(4) ) 3794 Hz, 1JPt(2),Pt(1) ) 1340 Hz) ppm. Reaction of 1 with para-H2. For the PHIP experiments, hydrogen enriched in the para spin state was prepared by storing H2 over Fe2O3 at 77 K for 3-4 h.53 1 H para-hydrogen spectra were performed on a JEOL EX-400 spectrometer operating at 399.65 MHz. All reactions with para-H2 were carried out in a 5 mm NMR tube equipped with a J. Young valve. ALTADENA and PASADENA experiments were performed in order to detect hyperpolarized signals. In the ALTADENA experiment 6.8 bar of para-H2 was added to frozen solutions of the metal complex (0.4 mL, 50 mM); the sample was warmed, shaken, and introduced into the magnet, where 1H spectra were recorded by a single scan using a 45° pulse. In the PASADENA experiment the solution containing the sample under investigation (0.4 mL, 50 mM) was introduced into the magnet and the gas supplied through a capillary inserted into the NMR tube. NMR spectra (16 scans) were acquired 1-2 s after stopping the gas flow. rf pulses of 45° were used to maximize the hyperpolarized PASADENA signal. Deuteration of 1. The deuterated compounds 2-d2 and 3-d2 were prepared in yields higher than 95% by following the procedures described above for the synthesis of their unlabeled analogues 2 and 3, using dry D2 in place of H2. Main NMR features of 2-d2 are as follows. 2H NMR (CH2Cl2, 298 K): δ -1.4 (m, t-D, 2JD,P(1) ) 18 Hz, 2JD,P(2) ) 4 Hz, 1JD,Pt(1) ) 150 Hz), -4.2 (m, µ-D, 2JD,P(2) ) 10 Hz, 2JD,P(4) ) 10 Hz,1JD,Pt(1) ) 88 Hz, 1JD,Pt(2) ) 95 Hz) ppm. 31P{1H} NMR (CH2Cl2, 298 K): δ 118.8 (d, P(1), 2JP(1),P(3) ) 244 Hz, 1JP(1),P(t1) ) 1587 Hz, 1JP(1),Pt(2) ) 1090 Hz), 73.5 (d, P(3), 2JP(3),P(1) ) 244 Hz, 1JP(3),Pt(2) ) 2462 Hz), 12.1 (br, P(4), 3JP(4),P(2) ) 49 Hz, 2JP(4),P(3) ) 21 Hz, 1JP(4),Pt(2) ) 3930), 9.6 (d, P(2), 3JP(2),P(4) ) 47 Hz, 1JP(2),Pt(1) ) 3931 Hz, 2 JP(2),Pt(2) ) 201 Hz) ppm. Main NMR features of 3-d2 are as follows. 2H NMR (CH2Cl2, 298 K): δ -5.4 to -6.0 (t-D + µ-D) ppm. 31P{1H} NMR (CH2Cl2, 298 K): δ 132.8 (pseudo t, P(1), 2JP(1),P(2) ) 256 Hz, 2JP(1),P(3) ) 260 Hz, 2JP(1),P(4) ) 16 Hz, 1JP(1),Pt(1) ) 2053 Hz, 1JP(1),Pt(2) ) 1206 Hz), 80.4 (d, P(3), 2JP(3),P(1) ) 260 Hz, 1JP(3),Pt(2) ) 2570 Hz), 16.0 (d, P(2), 2JP(2),P(1) ) 256 Hz, 1JP(2),Pt(1) ) 2560 Hz, 2JP(2),Pt(2) ) 10 Hz), 11.3 (br, P(4), 1JP(4),Pt(2) ) 3759 Hz) ppm. Dehydrogenation of 2. Solid 2 (20 mg) was put in a J-valve NMR tube, which was warmed at 323 K and put under high vacuum. After 3 h the system was cooled to room temperature, C6D6 was added, and 31P{1H} and 1H NMR spectra were recorded, revealing the formation of 1 (ca. 80% based on 31P) and a small amount (ca. 20%) of 3. Determination of the Kinetic Isotope Effect (KIE) for the Addition of H2 to 1. In an NMR tube, deuterium gas was bubbled at room temperature (flow rate 0.2 mL/min) through a solution of 30.0 mg of 1 dissolved in 0.3 mL of CD2Cl2. During this time 31P NMR spectra were recorded every 30 min for 6 h. Under identical conditions, dihydrogen was bubbled through a solution of 1 and (53) Canet, D.; Aroulanda, C.; Mutzenhardt, P.; Aime, S.; Gobetto, R.; Reineri, F. Concepts Magn. Reson., Part A 2006, 28A, 321.

ARTICLES

the 31P NMR spectra were periodically recorded. The KIE was determined to be the ratio of the reaction times for D2/H2 for the two reactions when the spectra appeared to be superimposable. By 31 P NMR (D2 addition) and 31P{1H} NMR (H2 addition) analyses, the complete consumption of 1 was evaluated to occur in 330 and 230 min, respectively. Thus, the D2/H2 isotope effect kH/kD was determined to be roughly 1.4. Isotopic Exchange of 2 in the Solid State. Solid 2 (31 mg) was put in a J-valve NMR tube under 1 bar of D2 and left under sonication at room temperature for 2 days. Then, D2 was vented and to the solid was added 0.5 mL of toluene under dinitrogen. The resulting solution was used for 2H NMR analysis, which revealed the formation of 3-d2. This can be explained either by isotopic exchange leading to 2-d2 followed by isomerization to 3-d2 or by isomerization to 3 followed by isotopic exchange. We have therefore put solid 3 under 1 bar of D2 under the same conditions reported above, but no deuteration of the compound was observed. This demonstrates the occurrence of isotopic exchange for 2 also in the solid state. Reactivity of 2 with PHCy2. Compound 2 (60.0 mg, 0.050 mmol) was dissolved in 0.4 mL of C6D6, and PHCy2 (0.040 mL, 0.20 mmol) was added, causing an immediate darkening of the solution. The 31 P{1H} NMR recorded after 20 min of stirring revealed the presence of cis-[(PHCy2)2(H)Pt(µ-PCy2)Pt(PHCy2)(H)(κP-P(O)Cy2)](Pt-Pt) (5) and of free PHCy2. The same complex is obtained when 3 is reacted with excess PHCy2. Complex 5 was characterized in solution due to difficulties encountered in separating it from unreacted PHCy2. Spectroscopic Features of [(PHCy2)2(H)Pt(µ-PCy2)Pt(PHCy2)(H){KP-P(O)Cy2}] (5). 1H NMR (C6D6, 298 K): δ -3.61 (m, H(1), 2 JH(1),P(1) ) 4 Hz, 2JH(1),P(2) ) 20 Hz,2JH(1),P(3) ) 175 Hz, 1JH(1),Pt(1) ) 947 Hz), 5.53 (m, H(2), 1JH(2),P(2) ) 358 Hz), 4.07 (m, H(3), 1JH(3),P(3) ) 310 Hz) -4.84 (m, H(4), 2JH(4),P(1) ) 24 Hz, 2JH(4),P(4) ) 15 Hz,2JH(4),P(5) ) 173 Hz,1JH(4),Pt(2) ) 1004 Hz), 3.76 (m, H(5), 1JH(5),P(5) ) 290 Hz) ppm. 31P{1H} NMR (C6D6, 298 K): δ 4.4 (dd, P(1), 2 JP(1),P(2) ) 226 Hz, 2JP(1),P(4) ) 307 Hz, 1JP(1),Pt(1) ) 2160 Hz, 1JP(1),Pt(2) ) 1402 Hz), 12.6 (d, P(2), 2JP(2),P(1) ) 226 Hz, 1JP(2),Pt(1) ) 2056 Hz), 12.4 (br, P(3), 1JP(3),Pt(1) ) 2026 Hz), 90.2 (d, P(4), 2JP(4),P(1) ) 307 Hz, 1J P(4),Pt(2) ) 2634 Hz), 26.2 (s, P(5), 1JP(5),Pt(2) ) 2052 Hz) ppm. 195Pt{1H} NMR (C6D6, 298 K): δ -5148 (ddd, Pt(1), 1JPt(1),P(1) ) 2160 Hz, 1JPt(1),P(2) ) 2056 Hz, 1JPt(1),P(3) ) 2026 Hz), -4908 (ddd, Pt(2), 1JPt(2),P(1) ) 1402 Hz, 1JPt(2),P(4) ) 2634 Hz, 1JPt(2),P(5) ) 2052 Hz) ppm.

Acknowledgment. We thank the Italian MIUR (PRIN 2007, project prot. 2007X2RLL2) and COST Phosphorus Science Network (PhoSciNet, project CM0802) for financial support and Dr. A. Sibahoui and Prof. T. Repo (University of Helsinki, Helsinki, Finland) for HR-MS analyses. Supporting Information Available: Figures and tables giving NMR spectra of compounds 2, 2-d2, 3, and 3-d2, the HR-MS spectrum of 3, Cartesian coordinates of compounds, intermediates, and transition states, gas-phase and solution energies for the heterolytic and homolytic hydrogenation of 1 and for the intramolecular isomerization of 2, hydrogen/deuterium kinetic isotopic effects, energy profile for the associative mechanism of the 2 f 3 isomerization, and optimized geometries of the intermediates F, G, TS3, and TS5. This material is available free of charge via the Internet at http://pubs.acs.org. JA909747R J. AM. CHEM. SOC.

9

VOL. 132, NO. 13, 2010

4765